Steels for armouring are well known and are generally characterised in having a predominantly tempered martensitic structure. Such martensitic armour steels have high strength and good ballistic performance properties, which enables the steel to resist the impact of a high velocity projectile. Armour steel alloys can have a variety of chemical compositions and through the years military and security specifications have been developed which mostly focused on improving the hardness and impact resistance properties, and also the yield and tensile strength of these various alloys. One of the main thrusts of these developments has been to lower the thickness of the armour plate in order to reduce the mass of armoured vehicles and body armour.
Armour steel plates are generally produced by producing a billet, whether through ingot or continuous casting processes, and then hot rolling the armour steel to a desired plate thickness. The hot rolled steel plates are allowed to cool down to room temperature, after which they are re-heated to approximately 800° C.-900° C. in a process called austenisation, during which the steel acquires a predominantly austenitic microstructure. The steel is then quenched by means of water, oil or platten, and subsequently tempered at approximately 200° C. to improve fracture toughness.
One drawback of this heat treatment process is that it is time consuming and involves significant costs to take the steel through the re-heating, quenching and tempering process steps. Also, advanced manufacturing facilities and equipment, and skilled labourers are necessary to execute the process steps, which further add to manufacturing costs. Moreover, quenching has a tendency to cause distortion of as-rolled armour steel plates if not executed under strictly controlled conditions.
Hitherto is has not been customary to produce armour steel by means of air-cooling alone. If known armour steel alloys undergo air-cooling alone after austenisation, ferrite and pearlite often form as normal products of the austenitic microstructure. It will be appreciated that ferrite and pearlite have poor ballistic properties. Therefore, to increase ballistic performance, the steel is normally quenched after austenisation so as to acquire a predominantly martensitic microstructure, which is a significantly harder microstructure, but a structure which unfortunately has poor toughness performance. Hence the subsequent tempering step to increase fracture toughness.
Although not done before, the applicant wanted to develop an air hardenable armour steel alloy with a martensite-residual austenite structure as well as good ballistic properties, by utilising the effect of alloying elements in producing the required structure upon air cooling alone and without the need for tempering. In so doing, armour plate might be developed which represents an improvement as far as cost of production, ballistic resistance and plate mass is concerned. Another significant benefit of this process route would be that wider plates could be produced within existing flatness specifications, since plate flatness is generally better after slow cooling compared to rapid cooling. Through extensive research and development the applicant believes that it is now able to produce a high-hardness steel alloy, suitable for armouring applications, and with increased mechanical and ballistic performance characteristics when compared to competing products on the market, but with the significant difference that it can, in a certain thickness range, be air-cooled only and does not need to be tempered.